Inertial Navigation Systems — Page 248, Lesson 293

We're starting a fresh topic now: Inertial Navigation Systems, and I want to walk you through the checks and error conditions that come with setting one up. This is where the precision of the system really shows itself, and where operator discipline matters just as much as the hardware. Let me set the scene first. The INS computer assumes it will normally operate coupled to the flight director or autopilot, so that across-track errors don't occur. That's the design philosophy — the system is built to fly the track precisely, not to wander off and then correct. Now, the first thing we have is the test function. Look at Figure 18.27 — it shows the function switch in the test position. When you select test, all the digits on the various displays illuminate, either showing a figure or letters. This lets the operator check that all the functions are operating. It's a simple lamp and segment test — you're verifying the displays themselves work before you trust them. Then we move to the manual and automatic system checks, and this is where the real substance is. At the initial setting-up stage, the start position must be fed into the INS computer with a high degree of accuracy. Let me explain why that accuracy matters so much. If the initial latitude is slightly in error, the platform will not remain earth-horizontal once the equipment is switched into navigation mode. Here's the mechanism: the torque motors tilt the platform to keep it level, but they're driven by computer calculations based on the latitude you entered. If that latitude is wrong, the torque motors tilt the platform at an inappropriate rate. So the platform drifts away from earth-horizontal. And for the same reasons, the platform will not remain directionally aligned with respect to north either. Both level and heading alignment degrade together. Now, if the initial latitude setting is grossly in error — not just slightly, but grossly — the system will detect the error and warn the operator. This is one of the principal functions of the warning annunciator on the CDU, the Control Display Unit, while the equipment is in the align mode. How does it sense this? The equipment compares the apparent drift and topple rates sensed by the rate gyros against the corrections being applied by the torque motors. If those don't correspond, it knows the latitude input is wrong. The rate gyros sense what the platform is actually doing; the torque motors apply what the computer thinks should happen. A mismatch means bad input data. Now contrast that with longitude. An incorrect operator input of longitude will not affect the stability of the platform at all. The platform doesn't care about longitude for its leveling or alignment. But the consequences are still real: the track and distance from the departure point to the first waypoint will be incorrectly computed. And here's the key point — all subsequent indications of longitude will be in error by the amount of the initial input error. It's a fixed offset that propagates through everything. Then we come to the most dangerous case: an incorrect input of the latitude and longitude of any of the waypoints. This has serious consequences. The INS will navigate very accurately between waypoints — the system itself is flawless — but it is incapable of detecting operator malfunctions. The book calls it "finger trouble." The system cannot tell that you typed a wrong coordinate; it just faithfully navigates to wherever you told it to go. So how do you protect against this? Two checks. First, the waypoints should be recalled from store onto the LED display and rechecked before flight. You pull them back out of memory and verify them visually. Second, you call up the initial great-circle track, shown as TK/GS, and the distances, shown as DIS/TIME, between consecutive waypoints. Then you compare these values against those shown on the flight log, or the flight progress log, or the flight plan. If the INS-computed track and distance match your pre-planned values, you've confirmed the waypoints are correctly inserted. So the whole picture here is: the INS is a precision instrument that trusts its inputs completely. Latitude errors destabilize the platform physically. Longitude errors corrupt your position readouts. Waypoint errors send you to the wrong place with no warning. The test function verifies the displays, and the manual checks verify the data. That's the discipline of operating this system properly.